The incidence and prevalence of HIV-1 infection in the worldwide pediatric population is rapidly increasing. The major mode of transmission is the vertical passage of HIV-1 from mother to infant. The majority of these infants will develop symptoms within the first two years of life. Many of the currently available antiretroviral therapies have a limited time span of utility due to toxicities or the emergence of resistance. The development of new drugs for use in the pediatric population is critical. Ritonavir is a competetive selective inhibitor of the HIV protease. It is currently used in adults with AIDS. This is a phase I/II, dose finding, open label study of HIV-infected infants and children, who will be statified by age into 2 dose cohorts. The subjects will be given ritonavir along with lamivudine(3TC) and zidovudine(ZDV). The primary objective is to assess the pharmacokinetics of ritonavir, alone and in combination with 3TC and ZDV and determine any potential age-related differences. Other objectives are to determine the suitable dose of ritonavir in combination with 3TC and ZDV. Antiretroviral activity and immunological effects will be studied. Plasma virus and lymphocyte kinetics will be modeled. The rapidity with which virus becomes resistant to ritonavir will be determined in vivo.